[PATCH v3 4/5] clocksource: Enable clocksource_cyc2ns() to cover big cycles

[Feng Tang]

Current clocksource_cyc2ns() has a implicit limit that the (cycles * mult)
can not exceed 64 bits limit. Jason Gunthorpe proposed a way to
handle this big cycles case, and this patch put the handling into
clocksource_cyc2ns() so that it could be used unconditionally.

Suggested-by: Jason Gunthorpe <jgunthorpe@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Feng Tang <feng.tang@xxxxxxxxx>
---
 include/linux/clocksource.h |   11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/include/linux/clocksource.h b/include/linux/clocksource.h
index aa7032c..1ecc872 100644
--- a/include/linux/clocksource.h
+++ b/include/linux/clocksource.h
@@ -274,7 +274,16 @@ static inline u32 clocksource_hz2mult(u32 hz, u32 shift_constant)
  */
 static inline s64 clocksource_cyc2ns(cycle_t cycles, u32 mult, u32 shift)
 {
-	return ((u64) cycles * mult) >> shift;
+	u64 max = ULLONG_MAX / mult;
+	s64 nsec = 0;
+
+	/* The (mult * cycles) may overflow 64 bits, so add a max check */
+	if (cycles > max) {
+		nsec = ((max * mult) >> shift) * (cycles / max);
+		cycles %= max;
+	}
+	nsec += ((u64) cycles * mult) >> shift;
+	return nsec;
 }
 
 
-- 
1.7.9.5

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